The present invention relates generally to radio signal processing, and more particularly to systems and methods for reducing distortion in rf signals and thus enhancing the fidelity and sensitivity of radio receivers.
Conventional radio receivers function by receiving an rf signal and preamplifying it, and then processing the signal using a superheterodyne structure. The superheterodyne structure, in its simplest configuration, includes a mixer oscillator which mixes the received signal down to an intermediate frequency (IF) signal. The IF signal is then sent through a bandpass filter and demodulated by an envelope detector to recover the information (colloquially referred to as xe2x80x9cbasebandxe2x80x9d) that is carried by the received rf signal.
Of importance to the present invention is the fact that rf signals are corrupted by environmental factors during transmission. Conventional superheterodyne structures attempt to correct for signal corruption by suppressing corruption-induced noise using filtering techniques. Unfortunately, such conventional filtering, whether using analog or digital techniques, suppresses both noise and useful signal, thereby reducing the fidelity of the receiver. In other words, although filtering improves the ratio between useful signal and noise (referred to as the signal-to-noise ratio, SNR), it typically reduces system fidelity and signal strength.
Further, during demodulation, the envelope detector of a conventional superheterodyne structure effectively demodulates only one-half cycle, for example, the positive half cycle, of the IF signal. Only one half of the signal need be used, since the information attached to the positive half cycle during transmission is identical to the information attached to the negative half cycle during transmission. Accordingly, the negative half of each cycle of the received rf signal is discarded by the envelope detector, and replaced with a mirror image of the positive half.
It happens, however, that either one of the positive or negative half of a cycle can be distorted asymmetrically from the other half. Consequently, in instances wherein the negative half of a cycle is relatively uncorrupted, but the positive half cycle is corrupted, the opportunity to use the xe2x80x9cbestxe2x80x9d half of a cycle is lost. Thus, the portion of a corrupted IF signal that is ultimately demodulated and output by the envelope detector statistically can be expected to be the corrupt half 50% of the time.
In light of the above discussion as recognized by the present invention, it would be advantageous to analyze both the positive and negative halves of an rf signal cycle and determine which half is the xe2x80x9cbestxe2x80x9d half, and then extract the useful signal from this xe2x80x9cbestxe2x80x9d half. As further recognized by the present invention, it would be advantageous to accomplish such analysis prior to the non-linear transformation of the rf signal to the IF signal during mixing by the oscillator. Stated differently, it would be advantageous to accomplish such analysis prior to mixing, since the mixing function causes certain data in the signal to be irrecoverable and therefore precludes identification of some distortion and corruption in the xe2x80x9ctruexe2x80x9d signal post-mixing. As still further recognized by the present invention, it would be advantageous to adjust signal gain and tuning xe2x80x9con the flyxe2x80x9d to account for transmitter frequency drift and for sometimes constantly changing received signal strength at the antenna.
Accordingly, it is an object of the present invention to provide a system and method for reconstructing a radio signal prior to mixing and demodulating the signal. Another object of the present invention is to provide a system and method for reconstructing a radio signal to improve the extraction of useful portions of the originally transmitted signal that had been corrupted. Yet another object of the present invention is to provide a system and method for reconstructing a radio signal which adjusts signal gain and tuning from the antenna on the fly. Still another object of the present invention is to provide a system and method for reconstructing a radio signal which is easy to use and cost-effective.
An electromagnetic waveform reconstruction device includes an analog to digital converter (ADC) that is electrically connectable to an antenna for receiving an analog electromagnetic signal therefrom and digitizing the signal. The ADC outputs the digitized electromagnetic signal to a digital signal processor (DSP), which in turn outputs a reconstructed electromagnetic signal in accordance with a predetermined reconstruction paradigm. As more fully discussed herein, the DSP is electrically associable with a mixer circuit for sending the reconstructed electromagnetic signal thereto for mixing and demodulating the signal.
Preferably, the electromagnetic signal is an rf signal, and the device further includes a digital to analog converter (DAC) for converting the reconstructed rf signal to an analog reconstructed rf signal, prior to sending the reconstructed rf signal to the mixer circuit. Alternatively, the DSP digitally mixes the reconstructed rf signal and outputs an intermediate frequency (IF) signal to a demodulator.
As envisioned by the preferred embodiment, the DSP includes reconstruction means for effecting method steps to implement the predetermined reconstruction paradigm. In accordance with the present invention, the method steps include receiving both a positive half and a negative half of the digitized rf signal, and then analyzing the positive and negative halves to identify distorted portions and undistorted portions thereof. At least some of the distorted portions are removed and replaced with respective replacement portions. Thereby, the reconstructed rf signal is produced, with each replacement portion being based on at least some of the undistorted portions.
In one presently preferred embodiment, a controller is electrically connected to the DSP. Also, a preamplifier filter circuit (PFC) is electrically connectable to the antenna and to the ADC for amplifying and filtering the analog rf signal from the antenna prior to sending the analog rf signal to the ADC. Moreover, the PFC is also electrically connected to the controller. Advantageously, the PFC includes a frequency bandpass filter for attenuating signals having a frequency not equal to a pass frequency, and the controller dynamically establishes the pass frequency.
Furthermore, in the presently preferred embodiment, the PFC includes an amplifier for increasing, by a gain factor, the amplitude of signals having the pass frequency. As intended by the preferred embodiment, the controller establishes the gain factor. To this end, the DSP outputs a gain adjust signal to the controller when the rf signal input to the DSP is characterized by an amplitude outside. of a predetermined amplitude range. Stated somewhat differently, the DSP generates the gain adjust signal when its input signal is characterized by distortions due to a weak or clipped signal, and the DSP generates the gain adjust signal by determining information content of the signal. In response to the gain adjust signal, and the controller dynamically establishes the gain factor based on the gain adjust signal. If desired, the device of the present invention can be combined with an electromagnetic signal transmitter.
In another aspect of the present invention, an rf receiver includes an antenna and a signal reconstruction circuit electrically connected to the antenna. Accordingly, the signal reconstruction circuit receives an analog rf signal from the antenna. Per the principles of the present invention, the signal reconstruction circuit generates a substantially undistorted reconstructed waveform. A mixer circuit is electrically associated with the signal reconstruction circuit for generating an intermediate frequency (IF) signal based on the reconstructed waveform, and a demodulator decodes useful information from the IF signal.
In yet another aspect, a computer-implemented method is disclosed for processing a transmitted electromagnetic signal to extract useful information from the signal. The present method includes receiving the electromagnetic signal and reconstructing it in accordance with a predetermined reconstruction paradigm, and then, after reconstruction, mixing and demodulating the electromagnetic signal to extract useful information therefrom.
In still another aspect, a computer program device includes a computer program storage device which is readable by a digital processing system. A program means is provided on the program storage device, and the program means includes instructions that are executable by the digital processing system for performing method steps for reconstructing an rf signal prior to mixing and demodulating the rf signal. The method steps advantageously include receiving both a positive half and a negative half of the rf signal, and analyzing the positive and negative halves to identify distorted portions and undistorted portions thereof. The method steps further include removing at least some of the distorted portions and replacing each with a respective replacement portion to thereby produce a reconstructed rf signal, with each replacement portion being based on at least some of the undistorted portions.
In another aspect of the present invention, a device is disclosed for dynamically preamplifying and filtering an rf signal from an antenna, prior to mixing and demodulating the signal to extract useful information from it. The device includes a controller and a preamplifier filter circuit (PFC) electrically connectable to the antenna and in electrical communication with the controller for amplifying and filtering the rf signal. Per the present invention, the PFC includes a frequency bandpass filter for attenuating signals having a frequency not equal to a pass frequency. Additionally, the PFC includes an amplifier for increasing, by a gain factor, the amplitude of signals having the pass frequency. The controller dynamically establishes/adjusts the pass frequency and gain factor, based on the signal amplitude and distortion.
The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: